Soluble Fiber vs. Insoluble Fiber: What's The Difference & Do I Need Both?

If you've read any nutrition-focused articles or browsed a grocery aisle in the last few years, you've probably come across some of the buzzy benefits of fiber (Fills you up! Aids digestion! Etc.!). But there's one thing about fiber that's not quite so advertised: the difference between soluble fiber and insoluble fiber. Seriously, what the heck is dietary fiber, and how do you make sure you're getting enough?

We talked to a handful of registered dietitians to get the lowdown on the different types of fiber (i.e., what they are and what they do) and compiled their best advice for making sure you get what you need every day.

Fiber, which is found exclusively in plants, is a type of complex carbohydrate that your body cannot fully digest. There are two broad categories of fiber: soluble (many are also considered viscous fibers) and insoluble (also called “bulking fiber”). Your body handles each type of fiber differently, and, because of that, they play different roles in your health. 

"Soluble fiber dissolves into a gel-like substance that is important to help capture and remove toxins and cholesterol from the body," explains registered dietitian Amy Shapiro, M.S., R.D., CDN. Insoluble fiber is not digested by the body or absorbed into the bloodstream. Instead, it forms the "weight" and "bulk" of stools in the body, acting as a broom of sorts by sweeping waste along and aiding evacuation.

Both forms can help you feel full by creating volume in the digestive tract that lets your brain know you've eaten, but each also has specific benefits tied to how your body handles them.

According to a 2019 Foods review, soluble fiber dissolves in water and slows down digestion1, controlling how and when certain nutrients, like carbohydrates, are absorbed in the body. This can help support healthy blood sugar levels by stopping big surges of glucose from entering the bloodstream at once.

According to the Journal of Cardiovascular Nursing, soluble fiber also bolsters heart health by decreasing total and LDL cholesterol levels3. Additionally, it's a great tool for achieving and maintaining a healthy weight. Soluble and highly viscous fibers block the absorption of fats and increase the viscosity4 of food in the digestive tract, which promotes satiety (i.e., makes you feel full for longer). 

But soluble fiber has some indirect effects, too. Soluble fiber that is also fermentable (not all soluble fibers are), also known as prebiotic fiber, is an excellent source of food for the bacteria that live in your gut. When these prebiotic, fermentable fibers get to the colon, the bacteria feed on them, creating short-chain fatty acids5, or SCFAs, like butyrate, acetate, and propionate (bioactive gut metabolites and a form of postbiotics). These SCFAs provide energy, support metabolism, regulate blood sugar levels, and bolster healthy inflammatory responses6.

Insoluble fiber, which doesn't dissolve in water and passes through the digestive system intact, has more of a laxative effect7.* When insoluble fiber moves through the digestive tract, it pulls water into the stool, making it softer and easier to pass. This helps promote healthy elimination, as well as overall digestive health and function.

Although there is more research on viscous (soluble) fiber and healthy body composition8, insoluble fiber may also help you maintain your desired weight. In one study from The American Journal of Clinical Nutrition, participants that swapped their low-fiber breakfast cereal for a cereal high in insoluble fiber9 had improved appetite regulation and ate fewer calories throughout the day.

Many high-fiber foods contain a combination of both soluble and insoluble fiber. For example, one avocado contains about nine grams10 of fiber—70% of that11 is insoluble fiber and the remaining 30% is soluble fiber.

That said, some mostly-soluble fiber foods include:

Some foods that are primarily sources of insoluble fiber are:

Women 50 and younger should be getting at least 25 grams per day (and at least 28 grams to 29 grams per day if pregnant or breastfeeding, respectively), while men under 50 should get at least 38 grams per day. As we age, our fiber needs decrease: At 51 and over, women need at least 21 grams of fiber per day and men need at least 30 grams per day. 

However, according to a 2017 scientific review from the American Journal of Lifestyle Medicine, the average American eats only 16 grams of fiber12 per day—a far cry from the recommended intake.

While there are no specific recommendations for how much of that should be soluble versus insoluble fiber, most health experts recommend including a variety of foods in your diet to make sure you're getting both types.

Low fiber intake can lead to problems like suboptimal bowel movements, extra weight, and sluggishness, but it can also create gut, heart, and immune health issues down the road. And since fiber is found in some of the most nutrient-dense foods (e.g., fruits, vegetables, whole grains, and legumes), falling short on your fiber needs could mean that you're missing out on other important nutrients, too.

RELATED: How Much Fiber You Need Per Day + How To Increase Your Intake

So, what's the best way to increase your daily fiber intake? 

"Add more plants to your diet," says Shapiro. "Try to incorporate plant foods (whole grains, nuts, seeds, fruits, and veggies) at every meal. That can be oats or a smoothie at breakfast, salad at lunch, trail mix or fresh fruit for a snack, and cooked veggies at dinner."

To make sure you're getting adequate amounts of both types of fiber, Nichole Dandrea, M.S., RDN, registered dietitian and author of The Fiber Effect, takes it one step further: "Shoot for at least three colors, if not more, per meal, choosing from green, red, purple, yellow, orange, blue, and tan," she says. To translate that into food, think broccoli, tomato, cabbage, squash, carrots, blueberries, and oatmeal, respectively. For example, a fiber-full meal might look like a grain bowl with farro, broccoli, radish, carrots, and red onion.

In addition to eating more vegetables, you can also make the foods you're already eating more fiber-rich with a few small tweaks. 

"An easy, convenient way to increase fiber in one's diet is using ground flaxseeds and chia seeds," Carrie Kirkland, R.D., L.D., a bariatrics dietitian at Morrison Healthcare, tells mindbodygreen. "They are both versatile ingredients with minimal texture and taste. Two tablespoons of chia seeds have eight grams of insoluble fiber and two tablespoons of ground flaxseeds contain four grams of fiber. If you're struggling with dietary fiber intake, try adding them to your yogurt, smoothies, protein shakes, or cottage cheese."

"Ideally, we want to get [fiber] from foods," says Shapiro. "If you have a hard time meeting that through whole grains, nuts, seeds, fruit, and veggies, then you can add a supplement."

Kirkland warns against flavored fiber powders that have a lot of added sugar (something that you definitely don't want). It's important to read your supplement labels and make sure you're only getting the good stuff.

Avoid any fiber supplements with the following ingredients, which are all fancy names for sugar:

Lucky for you, we’ve already curated a list of premium fiber supplements with clean ingredients and efficacious doses: Check out our roundup of the best fiber supplements!

Whether you aim to include more whole plant foods in your diet, add a daily fiber supplement to your regimen, or try a combination of both approaches, it's important to increase your fiber intake gradually over the course of a few weeks—especially if your diet is fairly low in fiber now. 

Upping your fiber intake slowly gives the bacteria in your gut time to adjust to the new food source. Eating too much fiber (especially soluble fiber) too quickly can create excess gas and lead to uncomfortable side effects, like bloat, stomach upset, and suboptimal bowel movements. 

"Try increasing fiber intake by five grams each week, and make sure to drink plenty of water," Dandrea suggests. "If you’re increasing your fiber intake with beans, be sure to soak them overnight first to reduce gas-forming compounds. Cooking them with a pressure cooker and a piece of kombu seaweed can also help to reduce the gas-forming compounds."

Soluble and insoluble fiber are both important components of a healthy diet. Soluble fiber helps balance blood sugar levels and keep you full, while insoluble fiber promotes digestive health and keeps you regular.* Yet, even though fiber has myriad health benefits, most people aren't getting close to enough. 

The best way to make sure you're meeting your needs is by including a variety of plant-based foods (like colorful veggies, nuts, and seeds) in your diet and adding a premium fiber supplement (like mbg’s organic fiber potency+) to your routine for additional support.

If your diet is really low in fiber now, make sure you're increasing your intake gradually (i.e., over the course of several weeks), so you don't experience any unwanted digestive side effects as your body adjusts.